Acoustic decoupler

ABSTRACT

An acoustic decoupler which isolates both shear and longitudinal acoustic waves comprises first and second layers of precompressed balsa wood. The first and second layers lie in substantially parallel planes. The unidirectional grain structure of each of the first and second layers is aligned with the substantially parallel planes, and the unidirectional grain structure of the first layer is aligned essentially perpendicular to the unidirectional grain structure of the second layer.

United States Patent Higgs 1 1 Mar. 27, 1973 54 ACOUSTIC DECOUPLER 1,817,086 8/1931 Lindsay et al. ....1s1 33 o Inventor: Roland w. gg Orchard Lake, 1,880,153 9/1932 Rosenzweig ..l8l/33 R Mich Primary Examiner-Stephen J. Tomsky [73] Assignee: Honeywell, Inc., Minneapolis, Minn. Atmmey [,amom Koontz at a].

[22] Filed: Apr. 26, 1972 [21] Appl. N0.: 247,706

[56] References Cited UNITED STATES PATENTS 1,549,320 Lundin 181/33 G 57 ABSTRACT An acoustic decoupler which isolates both shear and longitudinal acoustic waves comprises first and second layers of precompressed balsa wood. The first and second layers lie in substantially parallel planes. The unidirectional grain structure of each of the first and second layers is aligned with the substantially parallel planes, and the unidirectional grain structure of the first layer is aligned essentially perpendicular to the unidirectional grain structure of the second layer.

2 Claims, 3 Drawing Figures ACOUSTIC DECOUPLER REFERENCE TO RELATED PATENT I APPLICATIONS Reference should be made to a copending patent application, Ser. No. 225,054, filed Feb. 10, 1972, by R. W. Higgs, entitled Composite Acoustic Decoupler, which is assigned to the same assignee as this application.

BACKGROUND OF THE INVENTION This invention relates to an acoustic decoupler useful in sonar systems. In particular, it relates to a composite acoustic decoupler capable of decoupling both shear and longitudinal acoustic waves.

In the design of sonar devices, it is often desirable to isolate the transducer elements acoustically from each other and from the structure of the device. This isolation is normally accomplished by acoustic decoupling materials, which provide acoustic isolation through impedance mismatch and internal attenuation.

The characteristic impedance, pc, of any material may be represented as the product of the materials density p and sound velocity 0. For a free plane wave, this is also equal to the specific acoustic impedance. When the acoustic impedance of one material is the same as that of an adjacent material, the materials are said to be matched; when the acoustic impedances of the two materials are different, the materials are mismatched.

Two of the desired properties of an acoustic decoupler are first, an insertion loss of at least 30 db per centimeter through a combination of reflection and absorption; and second, stable acoustical and mechanical properties with temperatures from to 30C and with pressures or uniaxial stresses, or both, up to 10,000 psi.

In a copending patent application, Ser. No. 225,149,

' filed Feb.l0, 1972, by P. M. DAmico and R. W. Higgs,

entitled Underwater Acoustic Device, which is assigned to the same assignee as the present application, an acoustic decoupler material satisfying theserequirements is described. This material is balsa wood which has been precompressed between about 2,500 psi and about 20,000 psi. As described in the copending patent application, precompressed balsa wood is the only known acousticdecoupler material which has been able to satisfy the above-mentioned requirements, although a large variety of materials have been used as acoustic decoupler materials.

Despite the many advantages of precompressed balsa wood, it does have one disadvantage as an acoustic decoupler. Balsa wood has been found to have transverse isotropy. It is believed that this transverse isotropy is the result of the unidirectional grain structure of balsa wood. Experimental results indicate that the sound velocity of acoustic waves having particle motion normal to the grain structure is much less than those acoustic waves having particle motionparallel to the grain structure. In other words, the sound velocity along balsa woods fiber structure (or grain) is greater than 10 times the sound velocity in directions normal to the grain. Therefore,sound propagation in the direction of the fiber structure must be avoided in decoupling applications. This is achieved by orienting the precompressed balsa wood body such that the longitudinal acoustic waves impinging on the decoupler are essentially normal to the fiber structure of balsa wood. While this efiectively decouples longitudinal acoustic waves, shear waves can pass through the decoupler with very little absorption.

SUMMARY OF THE INVENTION The acoustic decoupler of the present inventiondecouples both shear and longitudinal acoustic waves. The first and second layers of precompressed balsa wood lie in substantially parallel planes. Each layer has a unidirectional grain structure which aligned with the substantially parallel planes. The unidirectional grain structure of the first layer is aligned essentially perpendicular to the unidirectional grain structure of the second layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows one embodiment of the acoustic decoupler of the present invention;

FIGS. 2A and 2B show top and cross sectional views of a sonar system including the acoustic decoupler of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 20,000 pounds per square inch. The first and second layers 10 and'12 liein substantially parallel planes. As can be seen, the z direction representing the unidirectional grain structure of each of the first and second layers is aligned with the. substantially parallel planes. However, the z direction of first layer 10 is aligned essentially perpendicular to the z direction in second layer 12. If desired, first and second layers 10 and 12 may be joined by an adhesive material such as-a rubber-type cement or an epoxy.

To achieve maximum decoupling of longitudinal waves, the thickness of each layer should be rat/8, where n is an odd integer and A is the acoustic wavelength in the balsa wood calculated at the midband frequency of the acoustic waves.

FIGS. 2A and 2B show top and cross sectional views of a sonar assembly in which the acoustic decoupler of the present invention is used. Transducers-20a and 20b typically are piezoelectric crystals. Rubber window 22 protects the transducer assembly and improves coupling and directivity of the sound waves. The

matching elements 240 and 24b are typically quarterv wavelength sections of aluminum. Since aluminum has a characteristic impedance which is almost the geometric mean of the impedances of a typical piezoelectric crystal and a rubber window, and since the thickness of each matching element is one quarter wavelength, aluminum matching elements 24a and 24b greatly improve port 28. As described previously, the orientation of the z direction of first and second layers and 12 causes decoupling of any polarization of shear waves. I

It is readily apparent to those skilled in the art that many modifications of the present invention are possible. It should therefore be understood that the invention is not to be limited by the embodiments shown, but only by thescope of that attached claims.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 1. An acoustic decoupler for decoupling both longitudinal and shear acoustic waves, the acoustic decoupler comprising first and second layers of balsa wood precompressed to between about 2,500 pounds per square inch and about 20,000 pounds per square inch, the balsa wood having a unidirectional grain structure, the first and second layers lying in substantially parallel planes, and the unidirectional grain structure of the first and second layers being aligned with the substantially parallel planes and being aligned essentially perpendicular to one another.

..2. The acoustic decoupler of claim 1 wherein each of the first and second layers has a-thickness essentially equal to n)\/8, where n is an odd integer and A is the acoustic wavelength in the balsa wood calculated at the midband frequency of the acoustic waves. 

1. An acoustic decoupler for decoupling both longitudinal and shear acoustic waves, the acoustic decoupler comprising first and second layers of balsa wood precompressed to between about 2,500 pounds per square inch and about 20,000 pounds per square inch, the balsa wood having a unidirectional grain structure, the first and second layers lying in substantially parallel planes, and the unidirectional grain structure of the first and second layers being aligned with the substantially parallel planes and being aligned essentially perpendicular to one another.
 2. The acoustic decoupler of claim 1 wherein each of the first and second layers has a thickness essentially equal to n lambda /8, where n is an odd integer and lambda is the acoustic wavelength in the balsa wood calculated at the midband frequency of the acoustic waves. 